Abstract. Expansion of palm oil and rubber production, for which global demand is increasing, causes rapid deforestation in Sumatra, Indonesia, and is expected to continue in the next decades. Our study aimed to (1) quantify changes in soil CO 2 and CH 4 fluxes with land-use change and (2) determine their controlling factors. In Jambi Province, Sumatra, we selected two landscapes on heavily weathered soils that differ mainly in texture: loam and clay Acrisol soils. In each landscape, we investigated the reference land-use types (forest and secondary forest with regenerating rubber) and the converted land-use types (rubber, 7-17 years old, and oil palm plantations, 9-16 years old). We measured soil CO 2 and CH 4 fluxes monthly from December 2012 to December 2013. Annual soil CO 2 fluxes from the reference land-use types were correlated with soil fertility: low extractable phosphorus (P) coincided with high annual CO 2 fluxes from the loam Acrisol soil that had lower fertility than the clay Acrisol soil (P < 0.05). Soil CO 2 fluxes from the oil palm (107.2 to 115.7 mg C m −2 h −1 ) decreased compared to the other land-use types (between 178.7 and 195.9 mg C m −2 h −1 ; P < 0.01). Across land-use types, annual CO 2 fluxes were positively correlated with soil organic carbon (C) and negatively correlated with 15 N signatures, extractable P and base saturation. This suggests that the reduced soil CO 2 fluxes from oil palm were the result of strongly decomposed soil organic matter and reduced soil C stocks due to reduced litter input as well as being due to a possible reduction in C allocation to roots due to improved soil fertility from liming and P fertilization in these plantations. Soil CH 4 uptake in the reference landuse types was negatively correlated with net nitrogen (N) mineralization and soil mineral N, suggesting N limitation of CH 4 uptake, and positively correlated with exchangeable aluminum (Al), indicating a decrease in methanotrophic activity at high Al saturation. Reduction in soil CH 4 uptake in the converted land-use types (ranging from −3.0 to −14.9 µg C m −2 h −1 ) compared to the reference land-use types (ranging from −20.8 to −40.3 µg C m −2 h −1 ; P < 0.01) was due to a decrease in soil N availability in the converted land-use types. Our study shows for the first time that differences in soil fertility control the soil-atmosphere exchange of CO 2 and CH 4 in a tropical landscape, a mechanism that we were able to detect by conducting this study on the landscape scale.

Abstract. Oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) plantations cover large areas of former rainforest in Sumatra, Indonesia, supplying the global demand for these crops. Although forest conversion is known to influence soil nitrous oxide (N 2 O) and nitric oxide (NO) fluxes, measurements from oil palm and rubber plantations are scarce (for N 2 O) or nonexistent (for NO). Our study aimed to (1) quantify changes in soil-atmosphere fluxes of N oxides with forest conversion to rubber and oil palm plantations and (2) determine their controlling factors. In Jambi, Sumatra, we selected two landscapes that mainly differed in texture but were both on heavily weathered soils: loam and clay Acrisol soils. Within each landscape, we investigated lowland forests, rubber trees interspersed in secondary forest (termed as "jungle rubber"), both as reference land uses and smallholder rubber and oil palm plantations as converted land uses. In the loam Acrisol landscape, we conducted a followon study in a large-scale oil palm plantation (called PTPN VI) for comparison of soil N 2 O fluxes with smallholder oil palm plantations. Land-use conversion to smallholder plantations had no effect on soil N-oxide fluxes (P = 0.58 to 0.76) due to the generally low soil N availability in the reference land uses that further decreased with land-use conversion. Soil N 2 O fluxes from the large-scale oil palm plantation did not differ with those from smallholder plantations (P = 0.15). Over 1-year measurements, the temporal patterns of soil N-oxide fluxes were influenced by soil mineral N and water contents. Across landscapes, annual soil N 2 O emissions were controlled by gross nitrification and sand content, which also suggest the influence of soil N and water availability. Soil N 2 O fluxes (µg N m −2 h −1 ) were 7 ± 2 to 14 ± 7 (reference land uses), 6 ± 3 to 9 ± 2 (rubber), 12 ± 3 to 12 ± 6 (smallholder oil palm) and 42 ± 24 (large-scale oil palm). Soil NO fluxes (µg N m −2 h −1 ) were −0.6 ± 0.7 to 5.7 ± 5.8 (reference land uses), −1.2 ± 0.5 to −1.0 ± 0.2 (rubber) and −0.2 ± 1.2 to 0.7 ± 0.7 (smallholder oil palm). To improve the estimate of soil N-oxide fluxes from oil palm plantations in this region, studies should focus on large-scale plantations (which usually have 2 to 4 times higher N fertilization rates than smallholders) with frequent measurements following fertilizer application.

Abstract. Expansion of palm oil and rubber production, for which global demand is increasing, causes rapid deforestation in Sumatra, Indonesia and is expected to continue in the next decades. Our study aimed to (1) quantify changes in soil CO2 and CH4 fluxes with land-use change, and (2) determine their controlling factors. In Jambi Province, Sumatra, we selected two landscapes on heavily weathered soils that differ mainly in texture: loam and clay Acrisol soils. At each landscape, we investigated the reference land uses: forest and secondary forest with regenerating rubber, and the converted land uses: rubber (7–17 years old) and oil palm plantations (9–16 years old). We measured soil CO2 and CH4 fluxes monthly from December 2012 to December 2013. Annual soil CO2 fluxes from the reference land uses were correlated with soil fertility: low extractable phosphorus (P) coincided with high annual CO2 fluxes from the loam Acrisol soil that had lower fertility than the clay Acrisol soil (P < 0.05). Soil CO2 fluxes from the oil palm decreased compared to the other land uses (P < 0.01). Across land uses, annual CO2 fluxes were positively correlated with soil organic carbon (C) and negatively correlated with 15N signatures, extractable P and base saturation. This suggests that the reduced soil CO2 fluxes from oil palm was a result of strongly decomposed soil organic matter due to reduced litter input, and possible reduction in C allocation to roots due to improved soil fertility from liming and P fertilization in these plantations. Soil CH4 uptake in the reference land uses was negatively correlated with net nitrogen (N) mineralization and soil mineral N, suggesting N limitation of CH4 uptake, and positively correlated with exchangeable aluminum (Al), indicating decrease in methanotrophic activity at high Al saturation. Reduction in soil CH4 uptake in the converted land uses compared to the reference land uses (P < 0.01) was due to decrease in soil N availability in the converted land uses. Our study shows for the first time that differences in soil fertility control soil–atmosphere exchange of CO2 and CH4 in a tropical landscape, a mechanism that we were able to detect by conducting this study at the landscape scale.

The potential of palm-oil biofuels to reduce greenhouse gas (GHG) emissions compared with fossil fuels is increasingly questioned. So far, no measurement-based GHG budgets were available, and plantation age was ignored in Life Cycle Analyses (LCA). Here, we conduct LCA based on measured CO 2 , CH 4 and N 2 O fluxes in young and mature Indonesian oil palm plantations. CO 2 dominates the on-site GHG budgets. The young plantation is a carbon source (1012 ± 51 gC m −2 yr −1), the mature plantation a sink (−754 ± 38 gC m −2 yr −1). LCA considering the measured fluxes shows higher GHG emissions for palm-oil biodiesel than traditional LCA assuming carbon neutrality. Plantation rotation-cycle extension and earlieryielding varieties potentially decrease GHG emissions. Due to the high emissions associated with forest conversion to oil palm, our results indicate that only biodiesel from second rotation-cycle plantations or plantations established on degraded land has the potential for pronounced GHG emission savings.

&lt;p&gt;&lt;strong&gt;The potential of palm-oil biodiesel to reduce greenhouse gas (GHG) emissions compared to fossil fuels is increasingly questioned. So far, no measurement-based ecosystem GHG budgets were available, and plantation age was ignored in Life Cycle Analyses (LCA). We conducted LCA based on measured CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O fluxes in young and mature Indonesian oil palm plantations. CO&lt;sub&gt;2&lt;/sub&gt; dominated the on-site GHG budgets in both the young and mature plantations. The young plantation was a carbon source (1012 &amp;#177; 51 gC m&lt;sup&gt;-2 &lt;/sup&gt;yr&lt;sup&gt;-1&lt;/sup&gt;), while the mature plantation was a carbon sink (-754 &amp;#177; 38 gC m&lt;sup&gt;-2 &lt;/sup&gt;yr&lt;sup&gt;-1&lt;/sup&gt;). LCA considering the measured fluxes showed higher GHG emissions for palm-oil biodiesel than traditional LCA assuming carbon neutrality. Plantation rotation-cycle extension and earlier-yielding varieties potentially decrease GHG emissions. Due to the high emissions associated with forest conversion to oil palm, our results indicate that no emission savings are achieved from biodiesel from first rotation-cycle oil palm plantations. Only biodiesel from second rotation-cycle plantations or plantations established on degraded land has the potential for pronounced GHG emission savings.&lt;/strong&gt;&lt;/p&gt;